ARM: dts: at91: sama5d2 Xplained: use DMA for UART3

Use DMA for UART3 as we have enough channels and to show how to
specify DMA use with serial nodes.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>